Inclusion Complexations of Levamisole and Cocaine With Molecular Baskets. Fluorescence Displacement Assay for Detection of Cocaine and Its Adulterant Levamisole in Aqueous Media.

We investigated inclusion complexation of levamisole (LEV+), a common adulterant of cocaine (COC+), with six molecular baskets 13--618- of increasing cavity depth and anionic charge at the portal. 1H NMR and mass spectrometry showed that 13- to 618- bind to LEV+ to form binary complexes in water, with the stability increasing across the series and highest for dendritic basket 618- (Kd = 10 µM). From 1H NMR magnetic shielding and OPLS4 calculations, we deduced that LEV+ inserts its benzene in the binding pocket of 13--618- while the protonated heterocycle remains near the anionic portal. The stability of [LEV⊂6]17- arises from C-H…π/cation…π/π-π contacts, ion pairing, and hydrophobic effect (ΔH°< 0, ITC). Dendritic basket 618- was subsequently used to develop a fluorescent displacement assay (FDA) for the detection of LEV+ with alizarin red S (ALZ-) as a fluorescence reporter. LEV+ displaced the dye to generate a fluorescence response suitable for its detection in water over circa 10-300 µM range. In contrast, COC+ bound more strongly to basket 618-, forming binary (Kd = 1 µM) and ternary complexes (1H NMR, calorimetry, and FDA). The binding mode of COC+ differs from that of LEV+, with the drug's cationic fragment occupying the basket's cavity.